1. Field of the Invention
The invention relates general to biomedical fasteners and in particular to fasteners having an expandable head for the internal stabilization of a fractured bone and/or tissue.
2. Brief Description of the Prior Art
A variety of techniques exist in the field of osteosynthesis for treating bone fractures. Many known techniques utilize bone screws and bone fixation plates wherein the bone screws are connected to the ends of the bone and the connection carrier bridges the fracture. The connection carrier can in particular be a bone plate, a marrowbone nail or a fixator. With this it is desirable, whilst adapting to the nature of the bone part to be connected, for the optical alignment onto the fragments or for compensating target errors, to be able to incorporate fasteners, such as bone screws, at different angles into the connection carrier.
Many bone screws have heads with a roughly hemispherical-shaped seat surface of which one seat surface in passage holes is allocated to a bone plate. If for example with a tibia fracture the two bone pieces must be connected to one another, the metallic bone plate is applied onto the set-up bone pieces. Thereafter the screws are rotated into the bones such that the seat surfaces of the screw heads and of the plate holes come to bear on one another and the plate is pressed against the bone. From this there results a connection of bone parts, bone plate and bone screws.
It however has been shown that a loosening of the connection of bone screws and bone plates can take place. One cause lays in the insufficient stability of the angle connections of bone screw and bone plate which are secured by friction forces between the screw head and the plate hole. To resolve this problem an angular stable connection of the bone screw and bone plate must be made.
Simple mid-shaft fractures of bones are readily treated by bringing the fracture surfaces together and holding them in the desired orientation with respect to one another through the use of splints, casts and the like. Bones in general have dense outer, strong cortical portions and interior, non-cortical portions that may include cancellous bone. At the ends of bones this strong cortical region is typically thinner and the underlying cancellous bone tends to be a fluid filled porous medium which provides more “motion” and dissipates greater energy with transmission.
Comminuted fractures and fractures involving the breakage of a bone into numerous bone fragments are more difficult to deal with since one must attempt to reposition each bone fragment in an orientation relative to each other bone fragment so that the fragments may knit together properly. For this purpose, physicians have often used metal plates that attach to the outer cortical surfaces of the bones and which utilize bone screws to hold the bone fragments in position.
Another method involves the use of cerclage procedures in which a wire is, in effect, wrapped about a broken bone (or the bone and bone plate) to hold the fragments in place, the cerclage wire occasionally penetrating through the bone. Reference is made to Johnson et al., U.S. Pat. No. 4,146,022. Yet another common method is fixation of fragments with splints which are internal to the bone's medullary cavity. These are classified as intramedullary rods or interamedullary fixation devices. These devices may be metallic or polymeric, and typically involve a means to affix the ends of the device to prevent motion of one or more of the bone fragments around the device. When metallic devices are used, screws, pins and sliding nails are used to achieve this fixation. Another method, taught in Berger, U.S. Pat. No. 5,658,310, involves anchoring the balloon portion of a balloon catheter in the medullary cavity at one end of a long bone having a transverse fracture, and stretching the remaining portion of the elastic catheter across the fracture interface within the bone to maintain the fracture interface in compression. It would appear that unless the elastic catheter traverses the precise center of the bone at the fracture site, compressive forces will be uneven across the fracture site. That is, the compressive forces on the side of the bone nearest the catheter will be greater than the compressive forces on the opposite side of the bone, generating an unwanted bending moment across the fracture site. Furthermore, a primarily compressive repair is not able to buttress multiple fragment or share loading as is required to stabilize comminuted fractures, limiting the usefullness of the method to a specific class of simple fractures.
Surgical procedures used to mount bone plates and cerclage elements to a bone often require supportive tissue that is normally joined to the bone to be cut from the bony tissue to enable direct visual access to the bone. With cerclage procedures, one must entirely encircle a bone in order to hold the bony parts together.
Procedures using bone plates and cerclage elements also often tend to interrupt blood flow to the damaged bone fragments, and thus hinder the healing process. Moreover, the use of rigid bone plates and intramedullary rods especially with locked screws can lead to stress shielding of the fracture site. It is well known (Wolffs law) that bone growth is stimulated when stress is applied. However, continuous, excessive pressure applied to a bone can cause unwanted resorption of bone at the pressure site. In order to promote healing of bone fractures, the fracture surfaces that are brought together during reduction of the fracture should be subject to cyclic or periodic compressive forces so as to stimulate the growth of new bone across the fracture interface without causing bone resorption. When a fracture interface is immobilized, as by a cast, the bone material that is deposited at the fracture interface may have a collagen fiber matrix that is random rather than aligned with the fiber matrix of bone on either side of the fracture, the healed fracture interface being weaker in tension than bone on either side of the interface.
Some bone fractures result in the production of many bone fragments, and proper reduction of the fracture requires the fragments to be carefully reassembled next to each other with their fracture surfaces in contact. Bone screws and bone plate devices commonly are used for this purpose. Using bone screw techniques, two bone fragments may be joined together, and these two fragments as a unit may be moved into approximation with a third fragment and joined to it, and so on. Fragments that are thus joined together by rigid screws cannot move with respect to other fragments, and mismatching of the fracture surfaces as the first several fragments are joined together can have a compounding effect, causing mal-union or non-union of fracture surfaces and resulting in far less than perfect bone fragment assembly and healing.
As such, articular and comminuted fractures generally require special attention to create a repair construct stable enough to allow early mobilization, but not configured and assembled in a manner which causes stress shielding.
Stress shielding results from force transfer through the implanted stabilization device verses the bone fragments. This situation is exacerbated when bone fragments are held apart by the fracture repair implants. Appropriate reduction of fracture fragments is more important when more rigid “locked” fixation devices are employed as excessive stress-shielding can result in a non-union. The optimal results are achieved when: (1) normal bone anatomy is reconstructed; (2) a portion of the physiologic force is directly transmitted through the bone; and (3) the bone fragments are reassembled and supported in a manner that the fragments, and particularly any articular surface fragments, move less than about 1 to 2 mm in the early post operative stages while callus and/or bone are being formed.
Successful use of flexible plating techniques in unstable fracture patterns is dependent, in part, on the use of a combination of devices such that each fracture fragment is stabilized via direct fixation, buttressing or force neutralization.
When dealing with plating systems the placement of multiple screws on each side of the fracture can distribute of loading between more than one screw on either side.
Stabilization of a fracture requires prevention of translation in all three directions and rotation about all three axes. Restraining a point solves translation but not rotation. Plates provide some rotational stability. The best mechanical advantage is obtained during fixation when plates (or screws) are not placed along the same axis.
A range of fasteners are needed create rigid constructs which can provide mechanical stability to an injured skeletal structure, and yet facilitate optimal healing. Among the commonly used stabilizers are internal and external fasteners such as headed bone screws, pitch differential bone screws, bone screws with lag fragments, bone screws and pegs which, when locked to plates or rods, buttress fragments, bone bolts, bone nails, bone pins, bone plates, rods, rod connectors, cables, wires, external and adjustable fixators, et cetera.
Infractures in metaphyseal and epiphyseal bone, a single-sided internal fixation construct using non-rigid connections to plates or rods can not provide sufficient stability to prevent undesirable motion under non-resistance loading or passive motion, let alone normal functional loads. This is particularly applicable when pathological bone or comminution is encountered, and in certain juxta articular impaction fractures (e.g. a die punch fragment), In many cases appropriate stability can only be achieved by use of constructs with a combination of features such as rigid or semi-rigid connections (locked pegs or screws, unilateral motion as in sliding compression hip screw or sliding spinal plates), or use of multiple plates and fasteners or nails and fasteners.
Aptus by Medartis International (Germany) provides a fastener that retains its position through a surface wedge fit, relying on surface friction. This device, however, provides minimal material interference to resist pull out or pull through.
In U.S. Pat. No. 6,955,677 Dahners discloses the use of spherical threads on screws which tap in to a softened or compliant region of a plate which has been “disposed” on the inside surface of the aperture to facilitate this tapping.